The degree of porosity in interstellar dust-grain material is poorly defined, although recent work has suggested that the grains could be highly porous. The presence of pores on the surface of grains is known to influence the optical properties of the dust, but it could also potentially have an effect on the dust-grain surface chemistry. Pores could increase surface area, enhance binding energies locally, and possibly trap molecules within pores after the ice mantle has built up on the grain. Using computational kinetics simulations, we examine how grain-surface chemistry and ice composition of interstellar dust grains are affected by high degrees of porosity in the grain material. Idealized three-dimensional dust-grains are constructed, atom by atom, with varying sizes and degrees of porosity. Using the off-lattice microscopic Monte Carlo chemical kinetics model, MIMICK, diffusive chemistry is simulated on the constructed surfaces under physical conditions appropriate to dark interstellar clouds. The build-up of ice mantles, which are mostly composed of water, on the surface of porous grains results in the pores becoming covered, leaving voids under the ice. Once the pores are completely covered, the chemical and structural behavior is similar to non-porous grains of the same size. The most prominent chemical effect of the high degrees of grain porosity is that the molecular hydrogen formed on the grain surface becomes trapped within the ices and voids inside the grain pores. Trapping in this way may indicate that other volatiles, such as inert gases not included in these models, could be trapped within dust-grain porous structures when ices begin to form.

Chemical kinetics simulations of ice chemistry on porous versus non-porous interstellar dust grains

ACS Spring Meeting 2021

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